Group III-V heterostructure devices having self-aligned graded contact diffusion regions and method for fabricating same

ABSTRACT

A lateral injection group III-V heterostructure device having self-aligned graded contact diffusion regions of opposite conductivity types and a method of fabricating such devices are disclosed. The device includes a heterojunction formed by a higher bandgap III-V compound semiconductor formed over a lower bandgap III-V compound semiconductor. The method of the present invention allows the opposite conductivity type diffusion regions to diffuse simultaneously and penetrate the heterojunction. This results in compositional mixing of the compound semiconductor materials forming the heterojunction in the diffusion regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Group III-V lateral injectionheterostructure devices, and more particularly, to a lateral p-i-nphotodetector having doped self-aligned graded contact diffusion regionsof opposite conductivity types which penetrate an abrupt heterojunctionformed by the upper two layers and a method for manufacturing same.

2. Description of the Prior Art

For both majority and minority carrier lateral injection heterostructuredevices, it is desired to have regions where an abrupt heterojunction ismaintained and regions where it is compositionally graded. Examples ofdevices where this is of use are optical devices such as photodetectorsand lasers and electronic devices such as metal-semiconductor fieldeffect transistors, heterostructure metal-semiconductor field effecttransistors and heterostructure field effect transistors. In suchdevices, the abrupt heterojunction is necessary in regions where it isdesired to prevent carriers from reaching the surface of the device toreduce the leakage current. The graded region is desired where eitherinjection or collection of carriers is required to occur which isgenerally associated with doped contact diffusion regions. A gradedregion is desired in a doped contact diffusion region because it willincrease the speed of the device due to carriers being efficientlycollected or injected by the graded diffusion regions. In order tofabricate a graded contact diffusion region in such heterostructuredevices, the diffusion region must penetrate a type I Group III-Vcompound semiconductor heterojunction in which the higher bandgap III-Vcompound semiconductor has its conduction band higher and valence bandlower than the corresponding conduction and valence band in the lowerbandgap III-V compound semiconductor. Due to a requirement of highcarrier lifetimes, small size and high quality ohmic contacts in suchdevices, ion implantation techniques are not suitable for formingcontact regions in Group III-V heterostructure devices. In addition, ionimplantation followed by annealing does not lead to grading ofheterostructures.

Various ohmic contacts which can be used to form diffusion regions inGroup III-V compound semiconductors have been developed. For example,U.S. Pat. No. 4,593,307 is directed to the formation of a molybdenumgermanide contact to n-type gallium arsenide. U.S. Pat. No. No.4,540,446 shows an n-type contact diffusion region formed by ionimplantation of an n-type dopant into a germanium film and a subsequentheating step diffuses the dopant into a gallium arsenide substrate. Anarticle by Tiwari, S., et al., entitled "Ohmic Contacts to N-GaAs withGermanide Overlayers", Tech. Dig. of IEDM, 115 (Dec. 1983) shows anohmic contact to n-GaAs which uses germanium as the diffusing dopantimpurity and molybdenum germanide as a contacting metallurgy.

U.S. Pat. No. No. 4,843,033 relates to a method for diffusing zinc intoGroup III-V heterojunctions having layers of a small bandgapsemiconductor material (GaAs) formed over a layer of a larger bandgapsemiconductor material (AlGaAs). Zinc tungsten silicide (ZnWSi₂) is usedas a contact and dopant source. During a rapid thermal anneal, the zincis diffused through two layers of doped GaAs to contact an n-doped layerof AlGaAs. Carriers are free to combine at the surface because the widebandgap material AlGaAs is below the narrow bandgap material GaAs.

Thus, there is a need to develop a lateral injection Group III-Vheterostructure having graded contact diffusion regions which penetratea heterojunction formed by a layer of a high bandgap III-V compoundsemiconductor overlying a layer of a low bandgap III-V compoundsemiconductor and a method for manufacturing such heterostructures.

SUMMARY OF THE INVENTION

The present invention is directed to lateral injection Group III-Vheterostructures having self-aligned graded diffusion regions and amethod for fabricating same. The method of the present inventioninvolves forming an undoped intrinsic layer of a Group III-Vsemiconductor compound on a Group III-V compound substrate. An upperlayer of a Group III-V semiconductor compound having a wider bandgapenergy than the intrinsic layer is then formed on the intrinsic layer.The upper layer and the intrinsic layer form an abrupt Type Iheterojunction. Both layers can be formed by well known epitaxialtechniques such as molecular beam epitaxy (MBE) or metal-organicchemical vapor deposition (MOCVD). A nitride layer is then deposited onthe three layer Group III-V heterostructure. The nitride layer ispatterned by conventional techniques to form first contact regions.Next, a first contact material which contains a dopant of a firstconductivity type is deposited in the first contact regions. Secondcontact regions are then formed by the same conventional techniques usedto form the first contact regions. A second contact material whichcontains a dopant of a second conductivity type is deposited in thesecond contact regions. The structure is then subjected to a rapidthermal anneal during which both dopants simultaneously diffuse into theupper layer and penetrate the intrinsic layer to form doped gradeddiffusion regions of opposite conductivity types.

The use of a wider bandgap upper layer on top of the lower bandgapintrinsic layer reduces recombination of both electrons and holes andleads to larger built-in voltages. This results in a low leakagecurrent. The compositional mixing of the heterointerface in thediffusion regions allows electrons and holes to be collected efficientlywhich results in a fast response of the device, a large bandwidth at lowbias conditions, a large responsivity with a large dynamic range, andsubstantially reduces the long time constant tails in the temporalresponse and the low-frequency gain. The use of diffusion not onlymaintains a large carrier lifetime but also allows for a self-alignedstructure with a corresponding simplification in the fabricationprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are cross-sectional views disclosing the fabrication of theGroup III-V lateral p-i-n heterojunction structure of the presentinvention at successive stages in accordance with the method of thepresent invention.

FIG. 4 is an energy band diagram of the structure of FIG. 3 in regionswhere diffusion has not occurred.

FIG. 5 is an energy band diagram of the diffusion region of FIG. 3 dopedwith a p-type dopant.

FIG. 6 is an energy band diagram of the diffusion region of FIG. 3 dopedwith an n-type dopant.

FIG. 7 is a graph showing the current v. voltage characteristic of thephotodetector of FIG. 3 wherein the substrate, intrinsic layer and upperlayer are comprised of InP, Ga₀.48 In₀.52 As and Al₀.53 In₀.47 Asrespectively.

FIG. 8 is a graph showing the temporal response of the Ga₀.48 In₀.52 ASphotodetector of FIG. 7.

FIG. 9 is a graph showing the bias dependence of the Ga₀.48 In₀.52 Asphotodetector of FIG. 7.

FIG. 10 is a graph showing the responsivity of the Ga₀.48 In₀.52 Asphotodetector of FIG. 7 at two applied voltages.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a lateral p-i-n photodetectoris formed using a self-aligned graded contact diffusion process.Referring to the drawings, FIGS. 1-3 will be described in connectionwith the various steps of fabrication of the p-i-n photodetector of thepresent invention. While the method of the present invention will bedescribed in connection with forming a p-i-n photodetector, it should beunderstood that the features of the present invention may be adapted forother lateral injection heterostructures where it is desired to haveregions where an abrupt heterointerface is maintained and regions wherethe heterointerface is graded. Examples of such lateral injectionheterostructures are optical devices such as lasers and electronicdevices such as field effect transistors. It should also be understoodby those skilled in the art that various conventional processes relatingto applying, exposing and developing photoresist materials to formdesired patterns for masking layers are not specifically describedherein but are well known in the art. The present invention alsocontemplates the use of well known etching techniques such as reactiveion etching and plasma etching. In addition, the present inventioncontemplates the use of deposition techniques such as molecular beamepitaxy (MBE), metal-organic chemical vapor deposition (MOCVD) andplasma enchanced CVD (PECVD) that are also well known in the art and arenot specifically described herein.

Turning now to the drawings, FIG. 1 is a cross-sectional view of a GroupIII-V heterostructure 10 to which the method of the present inventioncan be applied. There is shown a semi-insulating substrate 12 of a firstGroup III-V compound semiconductor. An undoped intrinsic layer 14 of asecond Group III-V semiconductor is sandwiched between the substrate 12and an undoped upper layer 16 of a third Group III-V semiconductorcompound. Layers 14 and 16 can be grown by either MBE or MOCVD or anyother well known epitaxial technique. The intrinsic layer 14 may be inthe range 1 to 3 μm thick and upper layer 14 may be in the range of 200to 500 Angstroms thick. In one embodiment of the present invention thesubstrate 12, intrinsic layer 14 and upper 16 were comprised of GaAs,GaAs and Ga₀.7 Al₀.3 As respectively. In another embodiment of thepresent invention the substrate 12, intrinsic layer 14 and upper layer16 were comprised of InP, Ga₀.48 In₀.52 As and Al₀.53 In₀.47 Asrespectively.

As shown in FIG. 2, the next step is to deposit a nitride layer 18 onupper layer 16 by plasma enhanced chemical vapor deposition (PECVD), orany other suitable technique. Layer 18 has a thickness typically in therange of 500 to 2,000 Angstroms. A suitable nitride for layer 18 issilicon nitride.

Next, conventional photolithographic patterning and masking techniquesare used to define a contact line 20. The nitride layer 18 is thenetched typically by reactive ion etching (RIE) to form contact line 20.A first contact material 22 is then deposited in contact line 20 bysputtering or any other suitable technique. The patterned photoresistused to define line 20 is then lifted off. The thickness of thedeposited contact material 22 is in the range of 500 to 2,500 Angstroms.The first contact material includes a dopant of a first conductivitytype and a contact metal.

In the next step in accordance with the method of the present invention,conventional photolithographic patterning and masking techniques areused to define contact line 26. The nitride 18 is first etched by RIE toform contact line 26. Then a second contact material 28 is deposited incontact line 26 by sputtering or any other suitable technique. Thepatterned photoresist used to define line 26 is then lifted off. Thedeposited contact material 28 has a thickness in the range of 500 to2,500 Angstroms. The second contact material includes a dopant of asecond conductivity type and a metal.

Contact materials suitable for use in either of the above embodiments ofthe present invention include MoGe₂ and Zn doped tungsten (W(Zn)) havinga zinc concentration of between 1 and 5 percent. W(Zn) is used to formp-type contacts and p-type diffusion regions while MoGe₂ is used to formn-type contacts and n-type diffusion regions. Tungsten zinc silicide(WZnSi₂) is also a suitable contact material.

The structure is then subjected to a rapid thermal anneal tosimultaneously diffuse some of the dopants out of the first and secondcontact films respectively, and penetrate the heterojunction 32 to formdiffusion regions 34 and 36 respectively. The diffusion regions areheavily doped with said dopants and are of opposite conductivity types.The use of W(Zn) and MoGe₂ as first and second contact materials resultsin a p+ diffusion region 34 and an n+ diffusion region 36 respectively.The nitride layer 18 is used as a diffusion mask during the annealing.The rapid thermal anneal should be performed with a temperature rangefrom 650° C. and 750° C. and within a time period ranging from 1 to 300seconds. It is preferred that the annealing be carried out at 700° C.for 30 seconds.

The diffusion of Zn and Ge involves Group III lattice sites and henceresults in a compositional mixing of the Group III-V compoundsemiconductor of the upper layer 16 and intrinsic layer 14 in thediffusions regions 34 and 36. This results in a removal of the abruptheterojunction 32 in the diffusion regions 34 and 36. Thus, thediffusion regions 34 and 36 are graded. The layer distinctions betweenlayer 16 and 14 in the diffusion regions 34 and 36 are thus irrelevantand a dotted line is used only to show where the abrupt heterointerfaceused to be.

It is preferred that the thickness of diffusion regions 34 and 36 betwice that of the upper layer 16 to insure that the diffusion regions 34and 36 penetrate the heterojunction 32. Since the thickness of layer 16is typically between 300 and 500 Angstroms, the thickness of thediffusion regions 34 and 36 range from 600 to 1,000 Angstroms. Thespacing between the graded diffusion regions 34 and 36 which are alsoreferred to as "fingers", is typically in the range of 0.5 to 10 μm. Thedoping concentration in diffusion regions 34 and 36 is typically in therange of 10¹⁸ to 10¹⁹ cm⁻³ or greater.

Referring now to FIG. 4, there is shown an energy band diagram of layers14 and 16 of the photodetector of FIG. 3 in the regions betweendiffusion regions 34 and 36. The band arrangement of the second GroupIII-V compound semiconductor consists of conduction band edge 38 andvalence band edge 40 spaced apart by a corresponding bandgap. The bandarrangement of the third Group III-V compound semiconductor consists ofconduction band edge 42 and valence band edge 44 spaced apart by acorresponding bandgap. The bandgap energy of the third III-V compoundsemiconductor is higher than the bandgap energy of the second III-Vcompound semiconductor. In addition, the band alignment between thesetwo layers is a Type I alignment, namely, the band edges 38 and 40 ofthe smaller bandgap material are nested within the band edges 42 and 44of the larger bandgap material as shown in FIG. 4.

FIG. 5 shows the energy band diagram of the graded diffusion region 34doped with a p-type dopant. Since region 34 is heavily p-doped, the highhole conductivity of this layer ensures that the valence band 46 iseffectively flat. Due to the compositional mixing in diffusion region34, there is a gradual change in the conduction band 48. The gradeddiffusion region 34 allows holes to be collected efficiently by thep-type diffusion region 34.

Similarly, FIG. 6 shows the energy band diagram of the graded diffusionregion 36 doped with an n-type dopant. Since region 36 is heavilyn-doped, the high electron conductivity of this layer ensures that theconduction band 50 is effectively flat. Since the n-type diffusionregion 36 is graded, there is a gradual change in the valence band 52.The graded diffusion region 36 allows electrons to be collectedefficiently by the n-type diffusion region 36.

The lateral p-i-n photodetector of FIG. 3 is analyzed in FIGS. 7 to 10.The analyzed photodetector was comprised of a semi-insulating InPsubstrate, a Ga₀.48 In₀.52 As intrinsic layer and an Al₀.53 In₀.47 Asupper layer. W(Zn) was used for p-type contacts and graded diffusionregions and MoGe₂ was used for n-type contacts and graded diffusionregions.

FIG. 7 shows the current v. voltage characteristic of the Ga₀.48 In₀.52As photodetector. As shown in FIG. 7 the Ga₀.48 In₀.52 As photodetectorhas a low reverse leakage current. The low reverse leakage current isdue to the use of a wide bandgap Group III-V semiconductor overlying asmall bandgap Group III-V semiconductor which prevents surfacerecombination of carriers.

FIG. 8 is a graph showing the temporal response of the Ga₀.48 In₀.52 Asphotodetector with FWHM ranges between 31 and 35 ps for 3 to 6 V bias.The extracted bandwidth (BW) exceeded 18.0 GHz. The short long timeconstant tail due to compositional mixing in the diffusion regions canreadily be seen from this figure.

FIG. 9 is a graph of the bias dependence of the Ga₀.48 In₀.52 Asphotodetector. A direct bandwidth measurement as a function of bias wasperformed for two different finger spacings, 2 μm and 4 μm. The powerlevel was held constant at 100 μW. This figure shows that thephotodetector can be operated over a large bandwidth at low biasconditions. This is a result of the compositional mixing of the abruptheterojunction in the diffusion regions and the larger built in electricfields of p-i-n structures.

FIG. 10 is a graph of the responsivity of the Ga₀.48 In₀.52 Asphotodetector in Amps per Watt (A/W) as a function of light intensityfor two different bias voltages. The graded diffusion regions result ina large dynamic range of the responsivity as shown in FIG. 10. Inparticular, the responsivity is unchanged from -35 dBmW to 0 dBmW ofinput power.

In summary, the present invention results in a lateral p-i-nphotodetector where electrons and holes are collected efficiently due tothe graded diffusion regions. This results in a fast response, a reducedlong time constant in the temporal response, a large bandwidth at lowbias conditions and a large responsivity with a large dynamic range. Inaddition, the use of a higher bandgap III-V compound semiconductordisposed on a lower bandgap III-V compound semiconductor preventsrecombination of carriers by acting as an effective barrier to suchcarriers and leads to larger built-in voltages. This results in a lowleakage current. Transport in the photodetector of the present inventionis dominated by bulk effects with life times exceeding a microsecond.Bandwidths exceeding 18 GHz have been obtained for the Ga₀.48 In₀.52 Asphotodetector. For bias voltages in the 3 to 5 V range, the bandwidthsare well above 5 GHz. These voltages are now compatible with powersupply voltages of digital circuits. In addition, photodetectors formedby the present invention utilize material structures that are compatiblewith heterostructure based FET technologies, thus, making growthredundant.

While the invention has been particularly shown and described withrespect to illustrative and preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention which should be limited only bythe scope of the appended claims.

Having thus described the invention, what is claimed as new and what isdesired to be secured by Letters Patent is:
 1. A lateral p-i-nheterojunction device comprising:a substrate of a first Group III-Vcompound semiconductor material; an undoped intrinsic layer of a secondGroup III-V compound semiconductor material having a first bandgapenergy formed on said substrate; an undoped upper layer of a third GroupIII-V compound semiconductor material having a second bandgap energywhich is larger than said first bandgap energy formed on said intrinsiclayer, said upper layer forming a heterojunction with said intrinsiclayer; first and second contact materials disposed on the surface ofsaid upper layer in a spaced relationship; a first graded contactdiffusion region doped with a first conductivity type impurity extendingfrom a lower surface of said first contact material, a portion of saidfirst graded contact diffusion region penetrating said heterojunctionand extending into said intrinsic layer; and a second graded contactdiffusion region doped with a second conductivity type impurityextending from a lower surface of said second contact material, aportion of said second graded contact diffusion region penetrating saidheterojunction and extending into said intrinsic layer.
 2. The lateralp-i-n heterojunction device of claim 1 wherein said substrate is GaAs.3. The lateral p-i-n heterojunction device of claim 2 wherein saidintrinsic layer is GaAs and said upper layer is Ga₀.7 Al₀.3 As.
 4. Thelateral p-i-n heterojunction device of claim 1 wherein said substrate isInP.
 5. The lateral p-i-n heterojunction device of claim 4 wherein saidintrinsic layer is Ga₀.48 In₀.52 As and said upper layer is Al₀.53In₀.47 As.
 6. The lateral p-i-n heterojunction device of claim 1 whereinsaid first contact material is selected from the group consisting ofW(Zn) and ZnWSi₂.
 7. The lateral p-i-n heterojunction device of claim 1wherein said second contact material is MoGe₂.
 8. The lateral p-i-nheterojunction device of claim 1 wherein said first contact material isselected from the group consisting of W(Zn) and ZnWSi₂ and the secondcontact material is MoGe₂.
 9. The lateral p-i-n heterojunction device ofclaim 8 wherein said first conductivity type impurity is a p-typeimpurity and said second conductivity type impurity is an n-typeimpurity.